1. First open terminal by pressing Ctrl+Alt+T on keyboard. When it opens, run command to get root privilege:

sudo -i

2. Then add user gdm to access control list:

xhost +SI:localuser:gdm

3. Switch to user gdm in the console:

su gdm -l -s /bin/bash

4. In previous Ubuntu releases, you can now run gsettings command to change login screen preferences. But for Ubuntu 20.04, you’ll get a warnning says: “failed to commit changes to dconf: Cannot autolaunch D-Bus without X11 $DISPLAY”

To get pass it, run command:

export DISPLAY=:0

You may check the $DISPLAY value in another terminal window via echo $DISPLAY.

5. Finally, run command to disable user list:

gsettings set org.gnome.login-screen disable-user-list true

Finally restart your machine and enjoy!

To restore the changes, do the previous 4 steps in terminal and finally run command: